Turbo Tech: Compressor and Turbine Map Details

Turbo Tech: Compressor and Turbine Map Details

By Khiem Dinh

We have previously covered compressor wheel technology and also how compressor and turbine maps are generated.  Now we are going to dive a little deeper into the details of the compressor map and all-mysterious turbine map.  Furthermore, we are going to see how compressor wheel and turbine housing A/R changes affect turbine flow and efficiency.

The basic compressor map plots air flow and pressure ratio for a given compressor speed.  On the vertical axis is pressure ratio which is the compressor outlet pressure divided by the compressor inlet pressure.  We have mentioned this before, but what we neglected to mention was the type of pressure being measured; for compressor maps, we need to compare total pressures at the inlet and outlet of the compressor housing.  The total pressure of a flow is segregated into ambient, static and dynamic pressures.  We need to measure the total pressure due to different size piping changing velocities and therefore the dynamic and static pressures.

The pressure ratio calculated on compressor maps is generally a total-to-total pressure taking into account the ambient, static, and dynamic pressures.  We need to use total pressure due to Bernoulli’s principle that the total pressure is constant along a streamline.  Looking at the above diagram, let’s say that Area #1 is the compressor outlet and Area #2 is the larger diameter tube after the compressor outlet.  The dynamic pressure is directly affected by the velocity of the air and that is a function of the area of the tube it is flowing through.  Smaller area = higher velocity and vice versa.  Because the areas of #1 and #2 are different, the velocities are different therefore making the dynamic pressures different.  Because the dynamic pressures are different, the static pressures (the ones most often measured) are also different even though the total pressures are the same at #1 and #2.  So if only the static pressure is measured, the incorrect pressure ratio may be calculated.

Compressor maps also show a corrected mass flow and corrected compressor speed.  While the maps are generated in a controlled environment, the conditions will not be exactly the same every time.  Therefore, the mass flow rates and compressor speeds are corrected based on the environmental ambient pressure and temperature during testing.

Mass flow rate (Q) is corrected to reference values of temperature and pressure, much like how many chemistry calculations are done at STP(standard temperature and pressure).  By correcting every test to the same reference temperature and pressure allows the maps to be confidently compared.  The speed of the compressor (N) also needs to be corrected.

The turbine map is orientated differently than the compressor map with the pressure/expansion ratio on the horizontal axis and the corrected flow on the vertical axis; turbines are also known as expanders as the gas starts off at a high pressure entering the turbine and expand to a lower pressure as it exits the turbine.  Therefore, when speaking specifically about turbines, the pressure ratio is also known as the expansion ratio.  Back on topic to the turbine map, turbine efficiency also looks different as compared to the compressor map; there are efficiency lines which correspond to each compressor speed line with the efficiency values also plotted on the vertical axis.

Here is a real deal turbine map from Borg Warner.  The top set of red curves are the efficiency lines and the bottom set of green lines are the flow rates each labeled with their corresponding compressor speeds.  The turbine map also has corrected mass flow rates and corrected speeds just like the compressor map. However, one difference is how the turbine pressure ratio is calculated; the pressure ratio on the turbine side uses total pressure at the turbine inlet and static pressure at the turbine outlet.